This invention pertains to cathode ray tube (CRT) displays and more particularly to the generation of the synchronizing (sync) signals and blanking signals used in generating the raster scans of such displays.
It is common knowledge among designers of CRT display terminals that the vertical sweep rate of the display should be synchronized with the frequency of the power line. This follows from the fact that magnetic fields emanating from power supply transformers, flourescent fixtures, etc., invariably are in sufficient proximity to the display so as to affect the average position of the electron beam of the display. The overall effect is one in which the linearity of the display varies at a frequency equal to the difference between the sweep rate and the instantaneous power line frequency. This "slow motion" variation of the display is eliminated when the sweep rate of the display is synchronized to the instantaneous frequency of the interferring magnetic fields at the power line frequency.
A common method of achieving the required synchronization is through the use of phase-locked loop circuits in which a voltage controlled oscillator (VCO), operating at megahertz frequencies required by the system logic, is divided down to approximate the power line frequency (50 or 60 Hz.). This approximated frequency is then compared against the actual power line alternating current signal serving as the reference frequency in a phase detector. The phase detector generates an "error" signal which is used to adjust the VCO frequency such that the frequency divided down from the voltage controlled oscillator is synchronized to the power line frequency. Apart from being relatively expensive, this method requires some delicacy in design to insure that the circuits reestablish synchronization should strong noise force the phase-locked loop to temporarily lose control.
In addition phase-locked loop circuits operate over narrow ranges of the reference or power line frequency. Thus if a circuit is designed for 60 Hz alternating current power lines it cannot be used for 50 Hz power lines and one must design a new circuit.